Optical Coherence Tomography (OCT) is a technique for performing high-resolution cross-sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time (see, e.g., Huang et al. 1991). OCT is a method of interferometry that determines the scattering profile of a sample along the OCT beam. Each scattering profile in the depth direction (z) is called an axial scan, or A-scan. Cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse (x and y) locations on the sample. One of the principle advantages of OCT is its ability to image the various layers of the retina of the eye. Technical improvements in this modality permit data sets to be obtained in very short times. Doppler OCT can be used to determine flow velocity by measuring phase shifts between temporally separated measurements.
The ability to measure total retinal blood flow can provide an important diagnosis between healthy and pathological retinal tissue. Doppler OCT is applicable to the measurement of blood flow in the retina and in retinal vascular perfusion. The optimum area to measure total ocular blood flow is centered on the optical nerve head, as this is where most vessels enter and exit the retina. It is the current state of technology that even fast OCT systems have a difficulty acquiring temporally densely-sampled blood flow data of these vessels.
Important to proper determination of blood flow or velocities is knowledge of the angle or angular orientation between the vessel (or blood flow) and that of the beam probing the vessel (the Doppler angle). If these directions are perpendicular, then Doppler techniques, as historically practiced, yield no measurable shift. In cases where the Doppler angle is close to 90 degrees, accurate knowledge of the Doppler angle is critical to proper velocity determinations, because the absolute velocity is proportional to 1/cosine of the Doppler angle. There are, however, different OCT velocity measurement methods, using either phase or intensity variation measurements, which are influenced differently by the geometric arrangement between blood flow vector and the probe beam direction.
Doppler OCT has been successfully utilized in ophthalmologic diagnostic investigations as well as in functional imaging. (For a general introductory reference to Doppler OCT, see, e.g., Chen & Zhang 2008.) The recent advances in Fourier-domain OCT sensitivity and speed have been remarkable and have impacted the capabilities of Doppler OCT measurement capacity. The results can, however, be limited by various physical and technical reasons, such as phase de-correlation due to high velocity, Brownian motion of the fluid itself, phase wrapping, large or poorly determined Doppler angles, inappropriate scan timescales, and low spatial sampling as will be described in more detail below.
There are a variety of measurements that can obtain a frequency shift. Most of these involve measurement of phases or phase differences between normally adjacent A-scans and/or intensity or amplitude information or combinations of these various observables. There are sensitivity variations amongst these techniques, dependent upon the aforementioned geometric configuration, minimum and maximum velocities to be measured, temporal and spatial sampling, as well as variations in resolution and contrast. Other techniques used to determine a frequency shift include speckle variance, logarithmic intensity variance, differential logarithmic intensity variance, 2D correlation mapping, split spectra amplitude de-correlation, and filtering of the original fringe data. A popular technique, known as Doppler variance, uses autocorrelation which involves complex OCT data at each location (see, e.g., Zhao et al. 2000).
Circumpapillary Doppler OCT scans have been employed to provide quantitative information about the total retinal blood flow in vivo (see, e.g., Tan et al. 2012). They have typically used two concentric circular scans and determined a vessel's angle by measuring the position of its cross-section at two separate positions or locations. The drawback to this approach is that the centers of the vessel cross-sections for the two interleaved circular scans with different diameters have to be precisely detected within each individual scan. This is time consuming, especially as there has not been an automated method introduced so far. Furthermore, it is often difficult to define the center of the vessel cross-sections, leading to inaccuracies in the determination of the vessel's orientation relative to the scan or probing beam. This is of interest as small changes in this orientation with angles approaching orthogonality (in certain measurement styles) may lead to very large differences in blood flow.
The temporal resolution of the scans is limited by two factors. On the one hand, the sampling of the circumpapillary scan has to be sufficiently dense in order to avoid phase de-correlation. On the other hand, one may not arbitrarily increase the A-scan rate as this reduces the sensitivity for lower flow speeds.
Some of the constraining factors of Doppler OCT are outlined below: Phase noise: The phase noise limits the minimum quantifiable velocity. It is limited by two factors, the signal to noise ratio (SNR) and the oversampling factor. The SNR limited phase noise of an OCT system is described by (Park et al. 2005):
      Φ          err      SNR        =            1              SNR              .  
In most cases the phase noise is limited by the oversampling of the scanning system. Ideally the two measurements used to calculate the phase difference should be taken at the exact same location. In practice, one often uses scanning patterns, which don't allow 100% overlap between the two measurements. This causes phase de-correlation between the two measurements and, therefore, an increase in phase noise. The phase noise limited by the sampling density is described by (Park et al., 2005):
            Φ              err        sampling              =                                        4            ⁢            π                    3                ⁢                  (                      1            -                          exp              ⁡                              (                                                      -                    2                                    ⁢                                                            (                                                                        Δ                          ⁢                                                                                                          ⁢                          x                                                d                                            )                                        2                                                  )                                              )                      ,where Δx denotes the lateral distance between two measurements, and d the 1/e2 beam width at the focus. Besides the scanning pattern, sample motion may significantly contribute to the phase noise in a similar fashion.
Time difference between two measurements: The time between the two measurements used to calculate the phase difference influences the quantifiable velocity range. Larger time differences enable the detection of slower velocities, smaller time differences on the other hand shift the detectable velocity range towards higher speeds.
Fringe wash-out: Fringe wash-out is mainly a problem of spectral domain OCT (SD-OCT). The phase shift introduced by moving scatterers during the integration time of the camera, causes a blurring of the interference fringe signal. This wash-out causes a reduction in the interference fringe amplitude and therefore a reduction of SNR of the signal from moving scatterers. The fringe wash-out scales with the magnitude of the phase shift. For high velocities, the fringe wash-out may be so strong, that the SNR (signal-to-noise ratio) drops below one. Since as mentioned above, the phase noise is also limited by the SNR, this is a significant problem for Doppler OCT measurements.
Doppler angle: Doppler OCT is only sensitive to axial motion. In order to determine the absolute velocity of a moving scatterer, it is essential to know the angle between its velocity vector and the probing beam. A precise angle determination is paramount due to the secant term in the expression for velocity from phase shift (see, e.g., Chen & Zhang 2008 review article, op. cit.). Since most of the vessels in the inner retina lie almost perpendicular to the probing beam, already small errors in the Doppler angle may significantly influence the resulting absolute velocity measurement.